TITLE: GCN CIRCULAR NUMBER: 29055 SUBJECT: Chandra observations of GW170817 at ~3.3 years since merger (Full report) DATE: 20/12/16 13:28:30 GMT FROM: Aprajita Hajela at Northwestern U A. Hajela, R. Margutti, K. D. Alexander, W. Fong (Northwestern U.), E. Berger (Harvard), T. Laskar (Bath), T. Eftekhari (Harvard), D. Giannios (Purdue U.), P. K. Blanchard, R. Chornock, D. L. Coppejans (Northwestern U.), V. A. Villar (Columbia U.), P. Cowperthwaite (Carnegie), L. Sironi (Columbia), B. Margalit (Berkeley) report: We report on the complete Chandra data set of observations of GW170817 acquired at t=1209-1213 days since merger (IDs 22677, 24887, 24888, and 24889; PI Margutti; program 21510449) for a total exposure time of ~91 ks. Preliminary results from a subset of these observations have been reported by Hajela et al., GCN 29041, GCN 29019; Troja et al., GCN 29038. We confirm the presence of significant X-ray emission from GW170817 (significance of detection > 5 sigma, Gaussian equivalent) from the merged observations, with a source count rate of (0.9 +/- 0.3) e-4 cps (0.5-8 keV). From a joint spectral fit with a simple power-law spectral model using a photon index Gamma ~ 1.6, no intrinsic absorption NHint = 0 cm-2 (as found in e.g. Hajela et al. 2019), and Galactic neutral hydrogen column density NH,gal = 7.8e+20 cm-2 (Kalberla et al., 2005), we estimate an unabsorbed flux of 3.46(+1.06 -1.31) e-15 erg/cm2/s (0.3-10 keV, 1-sigma uncertainties), corresponding to a luminosity of 6.9 (+2.1 -2.6) e+38 erg/s at the distance of 40.7 Mpc (Cantiello et al., 2018). The observed emission is a factor of ~2 brighter than the first X-ray non-detection of GW170817 at ~2 days after merger. As previously reported in Hajela et al., GCN 29041, the inferred flux is in excess to the extrapolation of the off-axis jet model that best fits the broad-band afterglow observations at t<580 days (e. g., Hajela et al. 2019) and might represent the emergence of a new component of emission. An updated plot of the X-ray evolution of GW170817 can be found here: https://sites.google.com/u.northwestern.edu/synchrotron-emission-from-gw17/home We thank the entire Chandra team for scheduling and executing these observations. We thank the entire Chandra team for scheduling and executing these observations.